In recent years, hard disk drives have had considerably increased magnetic recording density. In such a trend, significant advances also have been made in reproduction magnetic head technology. Particularly, the use of a spin valve type magnetoresistive element (MR element) utilizing a giant magnetoresistive effect (GMR) allows the sensitivity of a magnetoresistive head (MR head) to be improved substantially.
The spin valve type MR element includes two ferromagnetic layers and a nonmagnetic layer interposed between the two magnetic layers. The magnetization direction of one of the ferromagnetic layers (pinned layer) is fixed by an exchange bias magnetic field generated from a magnetization rotation suppressing layer (pinning layer; this ferromagnetic layer and the magnetization rotation suppressing layer are referred to collectively as an exchange coupling film). The magnetization direction of the other ferromagnetic layer (free layer) changes according to an external magnetic field. As a result, a relative angle formed by the respective magnetization directions of the pinned layer and the free layer changes, and this change in relative angle is detected as a change in electric resistance.
The spin valve type MR element has, for example, a known configuration in which a Ni—Fe film, a Cu film and a Fe—Mn film are used as a magnetic layer, the nonmagnetic layer and the magnetization rotation suppressing layer, respectively. When these materials are used, a magnetoresistance change ratio (MR ratio) of about 2% is obtained (Journal of Magnetism and Magnetic Materials 93, p. 101, 1991). Since the use of FeMn as a material for the magnetization rotation suppressing layer results in a small MR ratio, and the corrosion resistance provided by FeMn itself is not sufficiently high, PtMn- and NiMn-based materials have been used for reproduction magnetic heads for hard disks. Further, it has been reported that an element in which an oxide such as NiO, α-Fe2O3 or the like is used for a magnetization rotation suppressing layer provides an MR ratio of 15% or higher.
Considering that further increases in magnetic recording density will be achieved, it is expected that existing GMR elements will reach their limits and result in shortage of output. With this in view, vigorous studies have been made on TMR (Tunnel Magnetoresistance) elements. Compared with the GMR elements, the TMR elements provide larger amounts of change in resistance, and the resistance itself of the TMR elements is considerably higher. In a TMR element, an insulation film of Al2O3 or the like is used as a nonmagnetic layer, and sensing is performed using a tunnel current passed in a direction perpendicular to a film plane.
However, the TMR elements have presented the following problem. That is, when an area of an element becomes extremely limited as magnetic recording density is increased, the resistance of the element becomes too high.